1. Field of the Invention
The present invention relates generally to communication systems. In particular, the invention relates to a method and system for restarting a reference clock of a mobile station after a sleep period with a zero mean time error.
2. Description of Related Art
A clock system that operates at a high frequency by utilizing a high frequency reference clock and which is adapted for use with a mobile station is known. One drawback of this type of clock system is that it uses a considerable amount of energy due to the fact that it continuously operates at a high frequency. Therefore, it is unsuitable for small and lightweight mobile stations, such as mobile cellular phones, which utilize a battery as a power source. In order to increase the life of the battery, so-called sleep clock systems have been developed.
When utilizing a sleep clock system, it is necessary for the mobile station to awaken its receiver to monitor a base station for message bursts at specified times, and to sleep during times when no messages are to be received. However, the amount of time that the receiver needs to be awake is small compared to the amount of time that the receiver can be asleep. During the sleep period, a low frequency low power sleep clock keeps track of time and the high frequency reference clock is deactivated and most system modules are off.
Message bursts are received at discrete intervals and last for an integer number of frames. The high frequency reference clock needs to be activated during these message bursts to synchronize the receiver with the incoming messages from the base station. More specifically, the receiver needs to be clocked at the high frequency reference clock frequency so that the receiver frames of the mobile station are synchronized with the incoming message frames from the base station.
One type of sleep clock system is disclosed in Finnish patent application No. 923976. This sleep clock system comprises two clock devices, the first of which operates at a high frequency and the second of which operates at a low frequency. The first clock device, the normal or reference clock, operates on the megahertz band and the second clock device, the sleep clock, on the kilohertz band. When the mobile station is in a rest condition, the first clock device is switched off and the second clock device with lower accuracy is employed instead. By this arrangement, the power consumption of the mobile station can be significantly decreased and the life of the battery correspondingly increased.
The sleep clock system described above is used in mobile stations of the GSM (Global System for Mobile Communications) system based on time division multiple access (TDMA). When the normal clock device of the mobile station""s sleep clock system is switched off, the receiver of the mobile station is arranged to receive incoming messages from the base station only in certain pre-defined time stamps termed pages. A page is a pre-defined time period in which the mobile station is prepared to recognize an eventual incoming message. The sleep period of the mobile station between two pages is relatively long, in the range of 1 to 2 seconds. This sleep period between pages needs to be accurately measurable. Unfortunately, the use of a low accuracy sleep clock for measuring this sleep period introduces timing errors.
Accordingly, there is a need for a method and system that minimizes the timing errors introduced by the use of a sleep clock in timing a sleep period for a mobile station and that restarts a high frequency reference clock after the sleep period with a zero mean wake-up time error to help ensure that the receiver of the mobile station is properly synchronized in time with a base station for the receipt of pages.
The present invention provides a method and system to minimize the timing errors introduced by the use of a sleep clock in timing a sleep period for a mobile station. Further, the present invention provides a method and system to restart a high frequency reference clock after the sleep period with a zero mean time error to ensure that the receiver of the mobile station is properly synchronized with a base station for the receipt of pages.
A low frequency low power sleep clock is calibrated with respect to a high frequency reference clock during a specified calibration period. The actual frequency of the high frequency reference clock during calibration is assumed to be known by the mobile station. The frequency is tied to the reference clock frequency of the base station. Based upon the calibration, a calibration error for the sleep clock is determined. In a preferred embodiment, the calibration error is determined by counting the actual number of reference clock cycles that occur during the calibration period and by subtracting the expected number of reference clock cycles for the calibration period from the actual number counted. The expected number of reference clock cycles that should be counted during the calibration period is a real number.
Next, a wake-up time error is calculated, based upon the calibration error, to compensate for the error of the sleep clock during the sleep period. In a preferred embodiment, the wake-up time error is calculated by multiplying the calibration error by the ratio of the sleep period to the calibration period. The wake-up time error is utilized to compensate for the error of the sleep clock such that the reference clock restarts at the end of the sleep period with a zero mean time error.
Advantageously, at the end of the sleep period, the reference clock of the mobile station will be restarted with a zero mean time error to provide reference clock cycles to system modules and will properly time synchronize the receiver of the mobile station with the base station for the receipt of pages. Further, because a low frequency low power sleep clock is utilized during the majority of the time between pages, the power consumption of the mobile station is decreased and the life of the battery utilized with the mobile station is correspondingly increased.
Other features and advantages of the present invention will be set forth in part in the description which follows and the accompanying drawings, wherein the preferred embodiments of the present invention are described and shown, and in part will become apparent to those skilled in art upon examination of the following detailed description taken in conjunction with the accompanying drawings, or may be learned by the practice of the present invention. The advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.